The present disclosure relates to a magneto-rheological fluid and a clutch using the same.
A magneto-rheological (MR) fluid is obtained by dispersing magnetic particles such as iron (Fe) particles, etc. in a dispersion medium such as oil, etc. The magnetic particles in the MR fluid suspend at random in the dispersion medium in the absence of a magnetic field. When the magnetic field is applied to the MR fluid, the magnetic particles form multiple clusters along a direction of the magnetic field, and yield stress of the MR fluid increases. Thus, rheological or mechanical properties of the MR fluid can easily be controlled by an electrical signal, and application of the MR fluid to various technical fields has been considered. At present, the MR fluid has been used in direct-acting devices such as shock absorbers for automobiles, seat dampers for construction machines, etc.
A magnetic fluid is another example of the fluid containing the magnetic particles dispersed in the dispersion medium such as oil, etc. The magnetic particles dispersed in the magnetic fluid have a diameter of about several nm to 10 nm, and the particles oscillate by Brownian motion caused by thermal energy. Thus, the magnetic fluid is completely different from the MR fluid in that the magnetic particles do not form the clusters even when the magnetic field is applied, and the yield stress does not increase.
The magnetic particles generally used in the MR fluid have an average particle diameter of several μm to several tens of μm. The magnetic particles in the MR fluid are larger than those in the magnetic fluid, and can form the clusters when the magnetic field is applied. However, the large magnetic particles in the MR fluid may precipitate when left standing, and may cause caking of the MR fluid. When the magnetic field is applied and released repeatedly, the magnetic particles are agglomerated, and a stable dispersion state cannot be maintained. For improved stability of the MR fluid, an MR fluid containing two types of the magnetic particles of different particle diameters has been studied (see, e.g., Japanese Translation of PCT International Application No. H07-507978 and International Patent Publication No. WO2012/120842).
For example, according to Japanese Translation of PCT International Application No. H07-507978, large diameter carbonyl iron particles and small diameter chromium dioxide particles are mixed. The chromium dioxide particles are adsorbed by the carbonyl iron particles to stabilize the MR fluid.
According to International Patent Publication No. WO2012/120842, large diameter carbonyl iron particles are mixed with a small amount of small diameter iron particles. This can stabilize the MR fluid, and can reduce shear stress under high shear rate conditions.